[PATCH] KVM: x86: fix singlestepping over syscall

[PATCH] KVM: x86: fix singlestepping over syscall

[Radim Krčmář]

From: Paolo Bonzini <pbonzini@redhat.com>

TF is handled a bit differently for syscall and sysret, compared
to the other instructions: TF is checked after the instruction completes,
so that the OS can disable #DB at a syscall by adding TF to FMASK.
When the sysret is executed the #DB is taken "as if" the syscall insn
just completed.

KVM emulates syscall so that it can trap 32-bit syscall on Intel processors.
Fix the behavior, otherwise you could get #DB on a user stack which is not
nice.  This does not affect Linux guests, as they use an IST or task gate
for #DB.

This fixes CVE-2017-7518.

Cc: stable@vger.kernel.org
Reported-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
---
 The patch is included in the following pull request to Linus.

 arch/x86/include/asm/kvm_emulate.h |  1 +
 arch/x86/kvm/emulate.c             |  1 +
 arch/x86/kvm/x86.c                 | 62 ++++++++++++++++++++------------------
 3 files changed, 34 insertions(+), 30 deletions(-)

diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
index 055962615779..722d0e568863 100644
--- a/arch/x86/include/asm/kvm_emulate.h
+++ b/arch/x86/include/asm/kvm_emulate.h
@@ -296,6 +296,7 @@ struct x86_emulate_ctxt {
 
 	bool perm_ok; /* do not check permissions if true */
 	bool ud;	/* inject an #UD if host doesn't support insn */
+	bool tf;	/* TF value before instruction (after for syscall/sysret) */
 
 	bool have_exception;
 	struct x86_exception exception;
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 0816ab2e8adc..80890dee66ce 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2742,6 +2742,7 @@ static int em_syscall(struct x86_emulate_ctxt *ctxt)
 		ctxt->eflags &= ~(X86_EFLAGS_VM | X86_EFLAGS_IF);
 	}
 
+	ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
 	return X86EMUL_CONTINUE;
 }
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 87d3cb901935..0e846f0cb83b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5313,6 +5313,8 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
 	kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
 
 	ctxt->eflags = kvm_get_rflags(vcpu);
+	ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
+
 	ctxt->eip = kvm_rip_read(vcpu);
 	ctxt->mode = (!is_protmode(vcpu))		? X86EMUL_MODE_REAL :
 		     (ctxt->eflags & X86_EFLAGS_VM)	? X86EMUL_MODE_VM86 :
@@ -5528,36 +5530,25 @@ static int kvm_vcpu_check_hw_bp(unsigned long addr, u32 type, u32 dr7,
 	return dr6;
 }
 
-static void kvm_vcpu_check_singlestep(struct kvm_vcpu *vcpu, unsigned long rflags, int *r)
+static void kvm_vcpu_do_singlestep(struct kvm_vcpu *vcpu, int *r)
 {
 	struct kvm_run *kvm_run = vcpu->run;
 
-	/*
-	 * rflags is the old, "raw" value of the flags.  The new value has
-	 * not been saved yet.
-	 *
-	 * This is correct even for TF set by the guest, because "the
-	 * processor will not generate this exception after the instruction
-	 * that sets the TF flag".
-	 */
-	if (unlikely(rflags & X86_EFLAGS_TF)) {
-		if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
-			kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 |
-						  DR6_RTM;
-			kvm_run->debug.arch.pc = vcpu->arch.singlestep_rip;
-			kvm_run->debug.arch.exception = DB_VECTOR;
-			kvm_run->exit_reason = KVM_EXIT_DEBUG;
-			*r = EMULATE_USER_EXIT;
-		} else {
-			/*
-			 * "Certain debug exceptions may clear bit 0-3.  The
-			 * remaining contents of the DR6 register are never
-			 * cleared by the processor".
-			 */
-			vcpu->arch.dr6 &= ~15;
-			vcpu->arch.dr6 |= DR6_BS | DR6_RTM;
-			kvm_queue_exception(vcpu, DB_VECTOR);
-		}
+	if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
+		kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 | DR6_RTM;
+		kvm_run->debug.arch.pc = vcpu->arch.singlestep_rip;
+		kvm_run->debug.arch.exception = DB_VECTOR;
+		kvm_run->exit_reason = KVM_EXIT_DEBUG;
+		*r = EMULATE_USER_EXIT;
+	} else {
+		/*
+		 * "Certain debug exceptions may clear bit 0-3.  The
+		 * remaining contents of the DR6 register are never
+		 * cleared by the processor".
+		 */
+		vcpu->arch.dr6 &= ~15;
+		vcpu->arch.dr6 |= DR6_BS | DR6_RTM;
+		kvm_queue_exception(vcpu, DB_VECTOR);
 	}
 }
 
@@ -5567,7 +5558,17 @@ int kvm_skip_emulated_instruction(struct kvm_vcpu *vcpu)
 	int r = EMULATE_DONE;
 
 	kvm_x86_ops->skip_emulated_instruction(vcpu);
-	kvm_vcpu_check_singlestep(vcpu, rflags, &r);
+
+	/*
+	 * rflags is the old, "raw" value of the flags.  The new value has
+	 * not been saved yet.
+	 *
+	 * This is correct even for TF set by the guest, because "the
+	 * processor will not generate this exception after the instruction
+	 * that sets the TF flag".
+	 */
+	if (unlikely(rflags & X86_EFLAGS_TF))
+		kvm_vcpu_do_singlestep(vcpu, &r);
 	return r == EMULATE_DONE;
 }
 EXPORT_SYMBOL_GPL(kvm_skip_emulated_instruction);
@@ -5726,8 +5727,9 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
 		toggle_interruptibility(vcpu, ctxt->interruptibility);
 		vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
 		kvm_rip_write(vcpu, ctxt->eip);
-		if (r == EMULATE_DONE)
-			kvm_vcpu_check_singlestep(vcpu, rflags, &r);
+		if (r == EMULATE_DONE &&
+		    (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
+			kvm_vcpu_do_singlestep(vcpu, &r);
 		if (!ctxt->have_exception ||
 		    exception_type(ctxt->exception.vector) == EXCPT_TRAP)
 			__kvm_set_rflags(vcpu, ctxt->eflags);
-- 
2.13.1

Re: [PATCH] KVM: x86: fix singlestepping over syscall

[Wanpeng Li]

2017-06-22 23:10 GMT+08:00 Radim Krčmář <rkrcmar@redhat.com>:
> From: Paolo Bonzini <pbonzini@redhat.com>
>
> TF is handled a bit differently for syscall and sysret, compared
> to the other instructions: TF is checked after the instruction completes,
> so that the OS can disable #DB at a syscall by adding TF to FMASK.
> When the sysret is executed the #DB is taken "as if" the syscall insn
> just completed.
>
> KVM emulates syscall so that it can trap 32-bit syscall on Intel processors.

We have a discussion to not expose syscall/sysret to Intel 32-bit
guest two years ago. https://lkml.org/lkml/2015/11/19/225 The
syscall/sysret just makes sense against long mode instead of
compatibility/legacy mode of Intel CPU. We will get a #UD in 32-bit
guest, and syscall emulation is introduced by commit 66bb2ccd (KVM:
x86 emulator: add syscall emulation) to handle it. So why we still
expose syscall/sysret to Intel 32-bit guest?

> Fix the behavior, otherwise you could get #DB on a user stack which is not
> nice.  This does not affect Linux guests, as they use an IST or task gate
> for #DB.
>
> This fixes CVE-2017-7518.
>
> Cc: stable@vger.kernel.org
> Reported-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> ---
>  The patch is included in the following pull request to Linus.
>
>  arch/x86/include/asm/kvm_emulate.h |  1 +
>  arch/x86/kvm/emulate.c             |  1 +
>  arch/x86/kvm/x86.c                 | 62 ++++++++++++++++++++------------------
>  3 files changed, 34 insertions(+), 30 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
> index 055962615779..722d0e568863 100644
> --- a/arch/x86/include/asm/kvm_emulate.h
> +++ b/arch/x86/include/asm/kvm_emulate.h
> @@ -296,6 +296,7 @@ struct x86_emulate_ctxt {
>
>         bool perm_ok; /* do not check permissions if true */
>         bool ud;        /* inject an #UD if host doesn't support insn */
> +       bool tf;        /* TF value before instruction (after for syscall/sysret) */
>
>         bool have_exception;
>         struct x86_exception exception;
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 0816ab2e8adc..80890dee66ce 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -2742,6 +2742,7 @@ static int em_syscall(struct x86_emulate_ctxt *ctxt)
>                 ctxt->eflags &= ~(X86_EFLAGS_VM | X86_EFLAGS_IF);
>         }
>
> +       ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
>         return X86EMUL_CONTINUE;
>  }
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 87d3cb901935..0e846f0cb83b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5313,6 +5313,8 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
>         kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
>
>         ctxt->eflags = kvm_get_rflags(vcpu);
> +       ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
> +

I guess this is used for "the sysret is executed the #DB is taken "as
if" the syscall insn just completed", however, there is no sysret
emulation, so how the #DB is taken after the sysret?

Regards,
Wanpeng Li

>         ctxt->eip = kvm_rip_read(vcpu);
>         ctxt->mode = (!is_protmode(vcpu))               ? X86EMUL_MODE_REAL :
>                      (ctxt->eflags & X86_EFLAGS_VM)     ? X86EMUL_MODE_VM86 :
> @@ -5528,36 +5530,25 @@ static int kvm_vcpu_check_hw_bp(unsigned long addr, u32 type, u32 dr7,
>         return dr6;
>  }
>
> -static void kvm_vcpu_check_singlestep(struct kvm_vcpu *vcpu, unsigned long rflags, int *r)
> +static void kvm_vcpu_do_singlestep(struct kvm_vcpu *vcpu, int *r)
>  {
>         struct kvm_run *kvm_run = vcpu->run;
>
> -       /*
> -        * rflags is the old, "raw" value of the flags.  The new value has
> -        * not been saved yet.
> -        *
> -        * This is correct even for TF set by the guest, because "the
> -        * processor will not generate this exception after the instruction
> -        * that sets the TF flag".
> -        */
> -       if (unlikely(rflags & X86_EFLAGS_TF)) {
> -               if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> -                       kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 |
> -                                                 DR6_RTM;
> -                       kvm_run->debug.arch.pc = vcpu->arch.singlestep_rip;
> -                       kvm_run->debug.arch.exception = DB_VECTOR;
> -                       kvm_run->exit_reason = KVM_EXIT_DEBUG;
> -                       *r = EMULATE_USER_EXIT;
> -               } else {
> -                       /*
> -                        * "Certain debug exceptions may clear bit 0-3.  The
> -                        * remaining contents of the DR6 register are never
> -                        * cleared by the processor".
> -                        */
> -                       vcpu->arch.dr6 &= ~15;
> -                       vcpu->arch.dr6 |= DR6_BS | DR6_RTM;
> -                       kvm_queue_exception(vcpu, DB_VECTOR);
> -               }
> +       if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
> +               kvm_run->debug.arch.dr6 = DR6_BS | DR6_FIXED_1 | DR6_RTM;
> +               kvm_run->debug.arch.pc = vcpu->arch.singlestep_rip;
> +               kvm_run->debug.arch.exception = DB_VECTOR;
> +               kvm_run->exit_reason = KVM_EXIT_DEBUG;
> +               *r = EMULATE_USER_EXIT;
> +       } else {
> +               /*
> +                * "Certain debug exceptions may clear bit 0-3.  The
> +                * remaining contents of the DR6 register are never
> +                * cleared by the processor".
> +                */
> +               vcpu->arch.dr6 &= ~15;
> +               vcpu->arch.dr6 |= DR6_BS | DR6_RTM;
> +               kvm_queue_exception(vcpu, DB_VECTOR);
>         }
>  }
>
> @@ -5567,7 +5558,17 @@ int kvm_skip_emulated_instruction(struct kvm_vcpu *vcpu)
>         int r = EMULATE_DONE;
>
>         kvm_x86_ops->skip_emulated_instruction(vcpu);
> -       kvm_vcpu_check_singlestep(vcpu, rflags, &r);
> +
> +       /*
> +        * rflags is the old, "raw" value of the flags.  The new value has
> +        * not been saved yet.
> +        *
> +        * This is correct even for TF set by the guest, because "the
> +        * processor will not generate this exception after the instruction
> +        * that sets the TF flag".
> +        */
> +       if (unlikely(rflags & X86_EFLAGS_TF))
> +               kvm_vcpu_do_singlestep(vcpu, &r);
>         return r == EMULATE_DONE;
>  }
>  EXPORT_SYMBOL_GPL(kvm_skip_emulated_instruction);
> @@ -5726,8 +5727,9 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
>                 toggle_interruptibility(vcpu, ctxt->interruptibility);
>                 vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
>                 kvm_rip_write(vcpu, ctxt->eip);
> -               if (r == EMULATE_DONE)
> -                       kvm_vcpu_check_singlestep(vcpu, rflags, &r);
> +               if (r == EMULATE_DONE &&
> +                   (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
> +                       kvm_vcpu_do_singlestep(vcpu, &r);
>                 if (!ctxt->have_exception ||
>                     exception_type(ctxt->exception.vector) == EXCPT_TRAP)
>                         __kvm_set_rflags(vcpu, ctxt->eflags);
> --
> 2.13.1
>

Re: [PATCH] KVM: x86: fix singlestepping over syscall

[Paolo Bonzini]

On 27/06/2017 05:41, Wanpeng Li wrote:
>> KVM emulates syscall so that it can trap 32-bit syscall on Intel processors.
>
> We have a discussion to not expose syscall/sysret to Intel 32-bit
> guest two years ago. https://lkml.org/lkml/2015/11/19/225 The
> syscall/sysret just makes sense against long mode instead of
> compatibility/legacy mode of Intel CPU. We will get a #UD in 32-bit
> guest, and syscall emulation is introduced by commit 66bb2ccd (KVM:
> x86 emulator: add syscall emulation) to handle it. So why we still
> expose syscall/sysret to Intel 32-bit guest?

Because you didn't post v2 of that patch, I guess. :)

>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 87d3cb901935..0e846f0cb83b 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -5313,6 +5313,8 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
>>         kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
>>
>>         ctxt->eflags = kvm_get_rflags(vcpu);
>> +       ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
>> +
>
> I guess this is used for "the sysret is executed the #DB is taken "as
> if" the syscall insn just completed", however, there is no sysret
> emulation, so how the #DB is taken after the sysret?

No, it's used for instructions other than syscall and sysret:

> +               if (r == EMULATE_DONE &&
> +                   (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
> +                       kvm_vcpu_do_singlestep(vcpu, &r);

syscall (and sysret if it were emulated) overwrite ctxt->tf with the
value of TF at the end of the instruction.  Other instructions don't, so
that singlestep depends on EFLAGS.TF before the instruction is executed.

Paolo

Re: [PATCH] KVM: x86: fix singlestepping over syscall

[Wanpeng Li]

2017-06-27 16:20 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>
>
> On 27/06/2017 05:41, Wanpeng Li wrote:
>>> KVM emulates syscall so that it can trap 32-bit syscall on Intel processors.
>>
>> We have a discussion to not expose syscall/sysret to Intel 32-bit
>> guest two years ago. https://lkml.org/lkml/2015/11/19/225 The
>> syscall/sysret just makes sense against long mode instead of

s/long mode/64-bit mode

>> compatibility/legacy mode of Intel CPU. We will get a #UD in 32-bit
>> guest, and syscall emulation is introduced by commit 66bb2ccd (KVM:
>> x86 emulator: add syscall emulation) to handle it. So why we still
>> expose syscall/sysret to Intel 32-bit guest?
>
> Because you didn't post v2 of that patch, I guess. :)
>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 87d3cb901935..0e846f0cb83b 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -5313,6 +5313,8 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
>>>         kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
>>>
>>>         ctxt->eflags = kvm_get_rflags(vcpu);
>>> +       ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
>>> +
>>
>> I guess this is used for "the sysret is executed the #DB is taken "as
>> if" the syscall insn just completed", however, there is no sysret
>> emulation, so how the #DB is taken after the sysret?
>
> No, it's used for instructions other than syscall and sysret:
>
>> +               if (r == EMULATE_DONE &&
>> +                   (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
>> +                       kvm_vcpu_do_singlestep(vcpu, &r);
>
> syscall (and sysret if it were emulated) overwrite ctxt->tf with the
> value of TF at the end of the instruction.  Other instructions don't, so
> that singlestep depends on EFLAGS.TF before the instruction is executed.

Why sysret is not emulated since SDM said that it can incur a #UD if
not in 64-bit mode?

Regards,
Wanpeng Li

Re: [PATCH] KVM: x86: fix singlestepping over syscall

[Paolo Bonzini]

On 27/06/2017 11:50, Wanpeng Li wrote:
> 2017-06-27 16:20 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>>
>>
>> On 27/06/2017 05:41, Wanpeng Li wrote:
>>>> KVM emulates syscall so that it can trap 32-bit syscall on Intel processors.
>>>
>>> We have a discussion to not expose syscall/sysret to Intel 32-bit
>>> guest two years ago. https://lkml.org/lkml/2015/11/19/225 The
>>> syscall/sysret just makes sense against long mode instead of
> 
> s/long mode/64-bit mode
> 
>>> compatibility/legacy mode of Intel CPU. We will get a #UD in 32-bit
>>> guest, and syscall emulation is introduced by commit 66bb2ccd (KVM:
>>> x86 emulator: add syscall emulation) to handle it. So why we still
>>> expose syscall/sysret to Intel 32-bit guest?
>>
>> Because you didn't post v2 of that patch, I guess. :)
>>
>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>>> index 87d3cb901935..0e846f0cb83b 100644
>>>> --- a/arch/x86/kvm/x86.c
>>>> +++ b/arch/x86/kvm/x86.c
>>>> @@ -5313,6 +5313,8 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
>>>>         kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
>>>>
>>>>         ctxt->eflags = kvm_get_rflags(vcpu);
>>>> +       ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
>>>> +
>>>
>>> I guess this is used for "the sysret is executed the #DB is taken "as
>>> if" the syscall insn just completed", however, there is no sysret
>>> emulation, so how the #DB is taken after the sysret?
>>
>> No, it's used for instructions other than syscall and sysret:
>>
>>> +               if (r == EMULATE_DONE &&
>>> +                   (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
>>> +                       kvm_vcpu_do_singlestep(vcpu, &r);
>>
>> syscall (and sysret if it were emulated) overwrite ctxt->tf with the
>> value of TF at the end of the instruction.  Other instructions don't, so
>> that singlestep depends on EFLAGS.TF before the instruction is executed.
> 
> Why sysret is not emulated since SDM said that it can incur a #UD if
> not in 64-bit mode?

"64-bit ring 0 to 32-bit ring 3" sysret ("sysretl") is supported by Intel:

	IF (operand size is 64-bit)
	THEN CS.Selector ← IA32_STAR[63:48]+16;
	ELSE CS.Selector ← IA32_STAR[63:48];
	FI;

If you want to add support for emulating sysret, in particular legacy
mode sysret, that would be okay.  You can extend the new testcase to run
in 32-bit mode, too.

Paolo

Re: [PATCH] KVM: x86: fix singlestepping over syscall

[Wanpeng Li]

2017-06-27 18:09 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>
>
> On 27/06/2017 11:50, Wanpeng Li wrote:
>> 2017-06-27 16:20 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>>>
>>>
>>> On 27/06/2017 05:41, Wanpeng Li wrote:
>>>>> KVM emulates syscall so that it can trap 32-bit syscall on Intel processors.
>>>>
>>>> We have a discussion to not expose syscall/sysret to Intel 32-bit
>>>> guest two years ago. https://lkml.org/lkml/2015/11/19/225 The
>>>> syscall/sysret just makes sense against long mode instead of
>>
>> s/long mode/64-bit mode
>>
>>>> compatibility/legacy mode of Intel CPU. We will get a #UD in 32-bit
>>>> guest, and syscall emulation is introduced by commit 66bb2ccd (KVM:
>>>> x86 emulator: add syscall emulation) to handle it. So why we still
>>>> expose syscall/sysret to Intel 32-bit guest?
>>>
>>> Because you didn't post v2 of that patch, I guess. :)
>>>
>>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>>>> index 87d3cb901935..0e846f0cb83b 100644
>>>>> --- a/arch/x86/kvm/x86.c
>>>>> +++ b/arch/x86/kvm/x86.c
>>>>> @@ -5313,6 +5313,8 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
>>>>>         kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
>>>>>
>>>>>         ctxt->eflags = kvm_get_rflags(vcpu);
>>>>> +       ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0;
>>>>> +
>>>>
>>>> I guess this is used for "the sysret is executed the #DB is taken "as
>>>> if" the syscall insn just completed", however, there is no sysret
>>>> emulation, so how the #DB is taken after the sysret?
>>>
>>> No, it's used for instructions other than syscall and sysret:
>>>
>>>> +               if (r == EMULATE_DONE &&
>>>> +                   (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)))
>>>> +                       kvm_vcpu_do_singlestep(vcpu, &r);
>>>
>>> syscall (and sysret if it were emulated) overwrite ctxt->tf with the
>>> value of TF at the end of the instruction.  Other instructions don't, so
>>> that singlestep depends on EFLAGS.TF before the instruction is executed.
>>
>> Why sysret is not emulated since SDM said that it can incur a #UD if
>> not in 64-bit mode?
>
> "64-bit ring 0 to 32-bit ring 3" sysret ("sysretl") is supported by Intel:
>
>         IF (operand size is 64-bit)
>         THEN CS.Selector ← IA32_STAR[63:48]+16;
>         ELSE CS.Selector ← IA32_STAR[63:48];
>         FI;

I see. :)

Regards,
Wanpeng Li